1. Field of the Invention
The present invention relates to a sliding member, and particularly to a sliding member which exhibits excellent wear resistance when operated in a water-based environment.
2. Description of the Related Art
Currently the drive force for industrial machines is mainly provided by oil hydraulics, but considering the environmental contamination and sanitary problems in the food industry due to runoff of working media (oil-based media), it is undesirable in some cases to use oil-based media as the working media. Using oil-based media in a garbage incinerator causes back flash. In recent years there has been research into replacing oil-based working media with water-based media, which are relatively harmless and non-flammable.
However, the following problems occur when working media are converted from oil-based media to water-based media. That is, because water-based media do not have the same lubricating effects as oil-based media, seizure may occur in sliding parts when water-based media are used in the operations of conventional industrial machines. The use of ceramics and engineering plastics as materials for sliding members has been proposed in order to prevent seizure, but this has not been practical because these materials are expensive in comparison with metal materials and have less workability or impact resistant than metal material.
Covering the base material constituting the sliding member with a hard coat has been proposed as a means of resolving above problems of using water-based media. For example, in Japanese Patent Application Laid-open No. H10-184692, it is proposed that using a metal material as the base material, a titanium nitride film be formed on the sliding surface of one member, while either the other member is made of a non-brittle material, or else a nitride ceramic, oxide ceramic or carbide ceramic is formed as a hard coat on the sliding surface of the other member. Even by forming the hard film described in Japanese Patent Application Laid-open No. H10-184692, however, it is difficult to further raise the lubricity and wear resistance in water-based environments so that seizure of the sliding member can be reliably prevented.
Moreover, in the case that high-pressure water is supplied inside machinery such as hydraulic pumps for example, because the contact pressure of the sliding member becomes same level as the water pressure, resistance to high surface pressure (adhesion between base material and coat) and low friction properties under high surface pressure are required, but conventional sliding members do not meet these requirements.
In Japanese Patent Application Laid-open No. 2003-26414, it is proposed that an amorphous hydrogen-free carbon film or a hydrogen-containing carbon film comprising a fixed amount of hydrogen be formed on a base material in order to achieve excellent wear resistance and strong adhesion with the base material in machine parts and the like. However, the coat disclosed here has a wide range of hardness values, i.e. Vickers hardness 10 to 100 GPa, and wear and damage are likely to occur quickly if the hardness of the coat is not controlled more strictly when coating a sliding member to be used in water-based environments. In Japanese Patent Application Laid-open No. 2000-128516, an amorphous carbon film having low-wear properties and excellent adhesiveness as a piston ring coat or the like is disclosed, but since this coat has a wide range of hardness values, i.e. Hv 2000 to 10000 (20 to 100 GPa given that HV 1000=10 GPa) in the high range, it is unlikely to reliably provide wear resistance of a sliding member to be used in water-based environments.